docs: def count_names_by_namespace(graph, namespace):
      """Get the set of all of the names in a given namespace that are in the graph.
  
      :param pybel.BELGraph graph: A BEL graph
      :param str namespace: A namespace keyword
      :return: A counter from {name: frequency}
      :rtype: collections.Counter
  
      :raises IndexError: if the namespace is not defined in the graph.
      """
      if namespace not in graph.defined_namespace_keywords:
          raise IndexError('{} is not defined in {}'.format(namespace, graph))
  
      return Counter(_namespace_filtered_iterator(graph, namespace))